Uranyl-activated strontium zinc pyrophosphate phosphors

ABSTRACT

Uranyl-activated strontium zinc pyrophosphate, with allowable substitution of minor amounts of barium for strontium and cadmium for zinc, efficiently produces green light having sharp emission peaks particularly suitable for reprographic work in monochrome copying of materials of various colors. Also, rare earth ions such as Eu 3 can be substituted for small amounts of strontium, preferably using K 1 for charge compensation. This adds red peaks to the emission.

United States Patent Hoffman 51 May 30, 1972 OTHER PUBLICATIONS Lei,Tsai- Teh et al., Polarographic Study of Uranyl- Pyrophosphate Complexin the Presence of Various Surface- [72] Inventor: Mary V. Hoffman,South Euclid, Ohio Assigneez General Electric p y Active Substances,Talanta, 1965, Vol. l2,pp. 269-276.

[22] Filed: Feb. 24, 1969 Primary Examiner-Carl D. Quarforth AssistantExaminer-P. A. Nelson [21] Appl' 805372 Attorney-John F. McDevitt, HenryP. Truesdell, Frank L.

Related U S, A li ti D t Neuhauser, Oscar B. Waddell and Joseph B.Forman [63] Continuation-impart of Ser. No. 710,391, Mar. 4,

1968, abandoned. [57] ABSTRACT Uranyl-activated strontium zincpyrophosphate, with allowa- [52] US. Cl. ..252/30l.1 R, 250/ 106 R blesubstitution of minor amounts of barium for strontium and [51] Int. Cl...C09k 1/36 d i f in fficiently produces green light having [58] Fleldofsearch ..25/30l.1,30l.2;264/O.5; sharp emission peaks particularlySuitable f reprographic 250/71 106 work in monochrome copying ofmaterials of various colors. 5 6 R t cted Also, rare earth ions such asEu can be substituted for small 1 e erences l amounts of strontium,preferably using K for charge com- UNITED STATES PATENTS pensation. Thisadds red peaks to the emission.

3,457,179 7/1969 Natansohn ..252/301.l 6 Claims, 2 Drawing Figures M95#223 x [W11 5/0,1 mm

firZn P 0 :00;

c u E E 2 1 4 E 1 11 3 1 a z 1 1 l 1 1 1 1 '1 I f 1/ 1 1 l J l 1 1 1 1 11 450 500 550 600 WflI/ELENGTH NHNOME TfES Zrz 5/0 Mn Patented May 30,1972 2 Sheets-Sheet 1 \QRSDNQE 299.32% MPKQQMQ WHVELENGTH-NHNOME TEES[mas/0N SPECTRA 0F Ti 1. srzn owua Zflz 5/04 MI? Inveni'or. Mara V.Ho++man by 2L Her A t lrorneg URANYL-AC'I'IVATED STRONTIUM ZINCPYROPI'IOSPI'IATE PI'IOSPHORS CROSS-REFERENCES TO RELATED APPLICATIONSThis application is a continuation-in-part of applicant's applicationSer. No. 710,391, filed Mar. 4, 1968, now abancloned.

BACKGROUND or THE INVENTION This invention relates to inorganiccrystalline luminescent materials. More particularly, it relates tostrontium zinc pyrophosphate luminescent materials.

In many inorganic crystalline luminescent materials, the spectralcharacteristics of the light produced, including the color, are similarfor certain activator ions, somewhat independent of the matrix. However,in other luminescent materials, the spectral characteristics of thelight produced by these same activators depends onthe matrix or on theprocessing given to the phosphor. Also the matrix-activator combinationcan have a very significant effect on brightness and othercharacteristics of light output, which, in turn, can have a controllingeffect on the commercial utility of the luminescent material orphosphor.

Many types of phosphate matrix systems have been investigated for use asluminescent materials. Binary orthophosphates and pyrophosphates ofcalcium, strontium, barium, cadmium and other elements with certainactivators are commercially useful as phosphors. Also, the uranyl ion, Uhas been investigated in various matrix systems, both organic andinorganic, and often containing water. However, organic andwater-containing systems generally are of limited commercial utility inlamp applications, because of their instability during normallamp-making processes. No lamp or electronic phosphors of commercialimportance are known in which the uranyl ion is present as an activator.

In general, activation with uranium appears to be of at least two types:that of U, and that of the uranyl ion, U0 in which the uranium also hasa formal valence of U. The first type is to be expected in compoundssuch as the tungstate, in which a direct substitution of U for W ispossible. This emission can be green and can contain broad, overlappinglines, but the fluorescence and the absorption spectra difi'erconsiderably from that found in the uranyl ion.

The spectra of the uranyl ion have been studied in considerable detailand can be identified by the consistent number of lines found atrepeating wavelength intervals, in both the absorption and thefluorescence spectra. The position and relative intensity of these linesare similar for all uranyl compounds, but the absolute intensity isquite dependent on the matrix. ln general, U0, fluorescence has beenfound in matrices in which the uranyl ion is a major component, andwhich also contain H O molecules, for example, Ca(U0,),( PO -Xl-I,O, themineral autunite. It is also found in solutions, in which the U0molecule retains its identity.

Among the early work on uranium activation in solids is that of M. K.Slattery (J. Optical Soc. of America, 19, 175, 1929). She examined alarge number of compositions with uranium as an activator and foundfluorescence in about a third of them. The results showed the phosphatesof cadmium, strontium and barium, each alone, to be inert, and of zinc,fluorescent only with large amounts of U0 It is not possible from thiswork to determine the compounds examined, the relative brightness found,or to determine if the fluorescence was due to U0 or to U. Thus, thenature of the emission cannot be determined, and phosphors generallyuseful in lamps were not taught by Slattery.

SUMMARY OF THE INVENTION The present invention in certain of itsembodiments provides as a luminescent material the crystalline compoundstrontium zinc pyrophosphate activated by the uranyl ion. This compoundpersists structurally with partial substitutions of barium for strontiumand cadmium for zinc, without materially decreasing the brightness ofthe material. Also, the compound accepts rare earth and alkaliadditions. Certain preferred embodiments of the invention provide aluminescent material wherein the constituents are present in approximatemolar amounts, measured as the oxides, of

A is (Sr, ,Ba,) with x from 0 to 0.4,

E is (Zn ,,Cd,) withy from 0 to 0.3,

Ln is at least one of Eu, Pr, Nd and Sm,

R is at least one of Na, Li, K and Rb,

a is from 0.95 to 1.05,

b is from 0.95 to 1.05,

c is from 0.98 to 1.02,

dis from 0.02 to 0.12,

e is from 0 to 0.020, and

f is from 0 to 0.020.

When In is used, e is preferably at least 0.0001.

A preferred embodiment of the invention for maximum U0, emissionprovides such a luminescent material in which at is from 0.15 to 0.25,

y isfrom 0.l0to0.15,

a is approximately 1.0,

b is approximately 1.0,

c is approximately 1.0,

d is from 0.04 to 0.08, and

e and fare 0.

In a preferred embodiment in which the visible sensitized emission ofthe rare earth is desired Ln is Eu R is K e is from 0.0010 to 0.010, and

fis from 0.00l0to0.010.

One optimum embodiment of the invention provides such a luminescentmaterial in which x is approximately 0.2,

y is approximately 0,

a is approximately 1.0,

b is approximately 1.0,

c is approximately 1 .0, and

d is approximately 0.06.

Since the exact situation of the U0, ion is not known, the phosphor willbe referred to herein by the conventional type of phosphor formula, forexample: SrZnP,0-,:UO However, the U0 content of phosphors of theinvention in solid solution in the matrix of the compound.

The optimum embodiment in which the rare earth emission is requiredwould depend on the desired ratio of the green U0 emission to the redEu" emission. The most intense Eu emission is obtained with value of eand f approximately 0.004. For optimum charge compensation and maximumEu emission, e and f should have the same values, however, the inventionis also operative when e and f are not the same and even without chargecompensation.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a graphical representation ofthe spectral distribution of light produced by SrZnlfiO zUO phosphorwhen excited by 2,537 angstrom unit (A) radiation, compared with that ofNational Bureau of Standards standard phosphor (NBS) No. 1028Zn,SiO,:Mn.

FIG. 2 is a similar drawing showing the emission spectrum ofSrZnP,O',:0.06UO,:0.001SEu with U0 and Eu emission peak linesidentified.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Work of the present inventionhas shown that the crystalline compound SrZnP,O which is distinct anddifferent from its end members Sr P O, and Zn P,O-,, will accept the U0:ion into its structure and fluoresce under ultraviolet light in thegreen region with the line emission typical of the uranyl ion. FIG. 1shows this emision and by comparison shows that the absolute peak heightof the two major lines is substantially higher than the height of NBSNo. 1028 Zn,SiO :Mn at the same wavelengths. For applications where highintensity like emission is required, such as for improved reprography ofmaterials of various colors, this emission of the material of thepresent invention is particularly useful and is superior to the NBSphosphor. Since materials cannot be copied well if the color of theilluminating light is the same as a principal color of the materialbeing copied, it is desirable to use light with one or more sharpemission peaks. Also, use of such sharp peaks avoids the indistinctresults that can be obtained with broad-band illumination.

A typical spectrum of phosphor of the invention containing Eu, shown inFIG. 2, is suitable for various purposes, having a more balanced whiteremission while still retaining the advantages gained from sharp emissionlines.

The broad numerical composition limits stated in the Sumis obtained whenthe values of a, b, and c are as close to 1.0 as

possible, and the value of d is between about 0.04 and 0.08. Up to 40mole percent Ba can be substituted for Sr before the brightness isdecreased, and with 15 to 20 mole percent substitution, the brightnessis increased. Up to 30 mole percent Cd can be substituted for Zn with nodecrease in brightness, and with 10 to 15 mole percent substitution, thebrightness is increased. Both Ba and Cd can be substituted for Sr and Znat the same time, and the brightness increase is retained.

Other partial substitutions, such as Ca for Sr, Cd for Sr, and Mg forZn, are possible in this matrix, but all of these result in a decreasein brightness. Such substitutions in small amounts, and othersubstitutions, which do not cause material decreases in brightness arewithin the scope of the present invention, although larger quantities ofthese and other impurities which do cause material decreases inbrightness are not within the scope of the invention.

The incorporation of the Ln rare earths Eu, Nd, Pr or Sm results in thesensitization and enhancement of the characteristic emission of the rareearth by the uranyl ion; Sm and Pr in the yellow-orange region, Eu inthe red and Nd in the infrared. Of these, the Eu has the highestintensity. The enhancement of the rare earth emission is a result oftransfer of energy within the phosphor crystal from the UOf tothe rareearth. The green emission from U0 is diminished by this transfer.Without the presence of U0 the rare earth essentially has no emission inthis matrix.

When Ln is Eu, R is preferably K.

The purpose of the incorporation of the R alkali cation Na, Li, K or Rbis to provide charge compensation for the Ln incorporation. Both cationssubstitute for Sr" according to the formula 2Sr Ln R. By this means, theLn emission is increased and the U0 emission is decreased over thatobtained when no alkali cation is incorporated.

The starting material which can be used are any source of Sr, Ba, Cd orZn which can be fired to the oxide or to a phosphate, such as SrCOSrHPO, ZnO, ZnNH PO Zn3(PO4)2'2H20, Baco g, BaHPO4, CdNH4PO With someformulations, a source of P 0 such as (N114); HPO4, is also required.The U0 activator ion can be supplied from any uranyl compound, such asuranyl acetate, UO (C H O -2H O. Alternatively, the starting materialscan include a coprecipitate of zinc uranyl ammonium phosphate asdescribed in Example 6 below.

The appropriate starting materials are mixed together and then fired ata temperature between 600 and 900 C. for several hours. They are thenmilled and fired again at 800 to 950 950C. for several hours. Anadditional milling and firing may be necessary to obtain the maximumbrightness.

EXAMPLE 1 Moles Material Grams 1.00 SrHPO, 183.6 1.00 ZnNHJO. 178.3 0.06U0,( Q11 0, ),-2H,O 25 .3

These materials are ball milled in acetone, dried and then screenedthrough mesh. These were then fired at 900 C. for 3 hours, ball milledagain and refired at 900 C. for 16 hours. All the firings were in aquartz or silica tray, in an air atmosphere. After the final firing, thephosphor is ground lightly and screened.

EXAMPLE 2 Moles Material Grams 1.00 SrHPO 183.6 1 .00 ZnO 81 .4 1.00(NI-1,),HPO, 132.2 0.06 UO,( C H O ),'2H O 25.3

These materials were blended together and fired at 600 C. for 2 hours.They were then ballmilled and retired at 900 C. for 16 hours. Afterfiring, the material is ground lightly and screened.

EXAMPLE 3 Moles Material Grams 0.80 SrHPO 146.5 0.20 BaHPO, 46.5 1.00ZnNH PO 178.3 0.06 UO (C H O '2H O 25.3

EXANIPLE 4 Moles Material Grams 1.00 SrHPO-, 183.6 0.90 ZnNH PO 1 60.50.10 CdNH PO 22.5 0.06 UO C H O ),'2H O 25 .3

EXAMPLE 5 Moles Material Grams 0.80 SrHPO. 146.5 0.20 Hal-1P0, 46.5 0.90ZnNH PO 1 60.5 0.10 CdNI-LPO, 22.5 0.06 UO C H,O=),'2H O 25 .3

To provide as a starting material the coprecipitate of the U0 activatorion in the ZnNl-LJO compound, Nl-LOl-l is added to an acid solutioncontaining the proper molar proportions of Zn", U0 and a slight excessof PO, ions. The exact composition or structure of the precipitate isnot known, but for the purpose of formula calculations it is assumed tobe Zn, ,,(UO) N1-LPO When this procedure is used, d can be up to atleast 0.12, whatever activator content is desired in the finishedphosphor. An amount of ZnO equal to d must be used to maintainstoichiometry.

EXAMPLE 6 Moles Material Grams ZnO Srl-lPO,

This coprecipitation method has the advantage of dispersing theactivator more thoroughly and of course can be used with dry mixing.Because of the more uniform distribution of the activator in thephosphor crystals, brightness increases of 10 percent to 15 percent canbe obtained by using this procedure. The incorporation of Ba and Cd asshown in Examples 3, 4 and 5 are also possible when the coprecipitatedprocedure is used.

Examples 3, 4, 5 and 6 are mixed either dry or in acetone and fired asExample 1.

The brightness of this phosphor was measured by comparing the intensityof the SrZnP,O, U peak at 522 nanometers (nm) with the peak at 523 nm ofthe NBS phosphor.

Phosphors prepared by Examples 1 and 2 have 100 to 110 W percent of thepeak intensity of the NBS phosphor. Phosphors prepared by Example 3 have140 percent of that peak intensity. Those prepared by Examples 4 and 5have about l30percent of that peak intensity. The position and therelative intensities of the U emission peaks are not changed by the substitutions shown in Examples 3, 4 and 5.

EXAMPLE 7 As an example of the incorporation of the preferred rare earthand alkali, the following could be used:

Moles Material Grams 1.00 Zn (U0 NHJO, 190.7 0.06 ZnO 4.88 1.0 Srl-llO183.6 0.002 E11 0, 0.704 0.002 loco, 0.276

TABLE I Effects of Eu and U0 Contents on Emission Composition Emission,peak intensity (nm) d e f 522 613 J' f') 0 0.01 0.01 0 0.5

The foregoing is a description of illustrative embodiments of theinvention which constitute substantial improvements over the prior art.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. As a luminescent material, phosphor crystals of strontium zincpyrophosphate activated by the uranyl ion which is dispersed in thematrix of said phosphor crystals.

2. A luminescent material according to claim 1 wherein the constituentsare present in approximate molar amounts, measured as the oxides, of

A is (Sr, ,Ba with x from 0 to 0.4,

E is (Zn, ,,Cd,,) withy from 0 to 0.3,

Ln is at least one of Eu, Pr, Nd and Sm,

R is at least one of Na, Li, K and Rb,

a is from 0.95 to 1.05,

b is from 0.95 to 1.05,

c is from 0.98 to 1.02,

dis from 0.02 to 0.12,

e is from O to 0.020, and

f is from O to 0.020.

3. A luminescent material according to claim 2 in which x is from 0.15to 0.25,

y is from 0.10 to 0.15,

a is approximately 1.0,

b is approximately 1.0,

c is approximately 1.0, and

d is from 0.04 to 0.08

4. A luminescent material according to claim 2 in which e is from 0.0010to 0.010, and

fis from 0.0010 to 0.010.

5. A luminescent material according to claim 2 in which x isapproximately 0.2,

y is approximately 0,

a is approximately 1.0,

b is approximately 1.0,

c is approximately 1.0, and

d is approximately 0.06.

6. A luminescent material according to claim 2 in which e and f are eachapproximately 0.004.

2. A luminescent material according to claim 1 wherein the constituentsare present in approximate molar amounts, measured as the oxides, ofa-(e+f) AO : b EO : c P2O7 : d UO2 : e/2 Ln203 : f/2 R20 wherein A is(Sr1 xBax) with x from 0 to 0.4, E is (Zn1 yCdy) with y from 0 to 0.3,Ln is at least one of Eu, Pr, Nd and Sm, R is at least one of Na, Li, Kand Rb, a is from 0.95 to 1.05, b is from 0.95 to 1.05, c is from 0.98to 1.02, d is from 0.02 to 0.12, e is from 0 to 0.020, and f is from 0to 0.020.
 3. A luminescent material according to claim 2 in which x isfrom 0.15 to 0.25, y is from 0.10 to 0.15, a is approximately 1.0, b isapproximately 1.0, c is approximately 1.0, and d is from 0.04 to 0.08 4.A luminescent material according to claim 2 in which e is from 0.0010 to0.010, and f is from 0.0010 to 0.010.
 5. A luminescent materialaccording to claim 2 in which x is approximately 0.2, y is approximately0, a is approximately 1.0, b is approximately 1.0, c is approximately1.0, and d is approximately 0.06.
 6. A luminescent material according toclaim 2 in which e and f are each approximately 0.004.